This invention relates to a control device for a hybrid vehicle which comprises an engine and an electric motor as a source of drive force, the electric motor being used as a generator.
Recently, the hybrid vehicle comprising an engine acting as a source of motive power for running the vehicle and an electric motor having a battery as a power supply has been developed. Tokkai Hei 10-201003 published by the Japanese Patent Office in 1998 discloses a hybrid vehicle having a generator in addition to an electric motor, the generator normally being driven in order to return the state of charge (SOC) of the battery to a fully charged level when the state of charge of the battery decreases.
However, in this type of hybrid vehicle, there is no provision for handling the battery charge of a deteriorated battery. As a result, when the battery consumes electric power, the state of charge is increased to substantially an initial fully charged level rather than the actual fully charged level. Unlike the battery in an initial state wherein there is no deterioration (i.e., unlike a brand-new battery), the deteriorated battery has the possibility of being overcharged when the control described above is used. This is because the fully charged level of a deteriorated battery is lower than an initial fully charged level.
Moreover, when a battery has deteriorated, the chargeable capacity of the battery while the vehicle is decelerating is eliminated as a result of the lowered chargeable level if a target state of charge is maintained at a high level in a non-deceleration driving state. This results in inefficient regeneration of energy.
Furthermore it is necessary to provide for battery deterioration in a hybrid vehicle provided with an idle-stop function which can automatically stop the engine when the vehicle is brought to a temporary halt and automatically restart the engine with the electric motor when predetermined conditions are fulfilled. This is due to the fact that when the battery deteriorates, a voltage drop occurs as a result of the increase in the internal resistance of the battery and therefore the electric motor cannot generate a sufficient output to start the engine.
It is therefore an object of this invention to avoid overcharging of a battery by controlling a battery state of charge in response to the actual chargeable capacity of a battery which is undergoing deterioration.
It is a further object of this invention to maintain a chargeable capacity of a battery and to increase the efficiency of regeneration of energy during deceleration.
It is yet a further object of this invention to guarantee stable vehicle restarting and to avoid shortfalls in the output of the electric motor when restarting the engine after an idle-stop operation.
In order to achieve above objects, this invention provides a control device for a hybrid vehicle, the vehicle having an engine (1) as a power source for running the vehicle and a motor/generator (2), and the control device comprising a high-voltage battery (11) which functions as a power source for the motor/generator (2), a low-voltage battery (14) for vehicle-mounted electrical loads which has a lower voltage than the high-voltage battery (11), an inverter (12) for controlling the motor/generator (2), a sensor (18,19) which detects a condition of the high-voltage battery (11); and a microprocessor (16).
The microprocessor is programmed to compute a state of charge (SOC) of the high-voltage battery (11) based on the condition of the high-voltage battery (11), compute a degree of deterioration of the high-voltage battery (11) based on the condition of the high-voltage battery (11), control a current generated by the motor/generator (2) so that the SOC of the high-voltage battery (11) coincides with a target SOC; and vary the target SOC according to the degree of deterioration of the high-voltage battery (11).
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.